The invention concerns a process for fractionating or refining natural lipid substances such as e.g. fats or oils by liquid/liquid extraction with a mixture of the two compressed gases propane and carbon dioxide.
When oils and fats are isolated on a technical scale from vegetable, animal or microbial natural substances either by extraction using solvents or mechanically by means of pressing, the crude product usually has to be requires purification in order to achieve an adequate quality. For purification various process steps are usually carried out within the framework of a so-called refining in order to separate undesired accompanying substances such as phospholipids (degumming) or dyes (bleaching).
Apart from these conventional working methods, for a long time processes have been described which use compressed gases to refine oils and fats (U.S. Pat. Nos. 2,660,590 and 2,548,434). According to the processes published in these patents the reverse solubility characteristics of compressed propane in the region of the critical state data (Tc: 96.degree. C., Pc=42 bar) are utilized to refine oils i.e. in particular to separate phospholipids and dyes.
The process according to DE-PS 10 69 629 is based on the same principle in order to achieve a fractionation of oily mixtures of higher molecular organic compounds. In DE-OS 42 22 153 compressed propane is also proposed under similar state conditions for deoiling oil-containing phospholipids.
Despite the relatively favorable process pressures and the comparably high solvent capacity, these processes generally have the disadvantage that they require relatively high process temperatures in order to achieve the desired extraction goal particularly when the content of the substances to be separated is relatively low such as e.g. in the case of phospholipids and dyes in the natural starting materials. In addition, the selectivity of propane near the critical region is too low in many cases to obtain satisfactory results.
In contrast to propane, supercritical carbon dioxide generally has a very high selectivity and has often been proposed for refining oils and fats (cf. for example Stahl, E., Quirin, K.-W., Gerard, D. "Verdichtete Gase zur Extraktion und Raffination, Springer Verlag, Berlin, 1988). However, the key problem in using supercritical carbon dioxide is the fact that it only has a solubilizing power for lipid materials at relatively high process pressures and thus the cost effectiveness of the corresponding processes is unsatisfactory in most cases.
In order to circumvent the disadvantages of pure supercritical carbon dioxide and compressed propane, it has already also been suggested in DE-OS 34 11 755 that a mixture of supercritical carbon dioxide and a subcritical entraining agent, in particular propane, is used to deoil crude lecithin. However, in this process modification the pressures are also still relatively high, namely between 50 and 800 bar and preferably between 80 and 200 bar.
Finally it is known from the German laid-open patent applications DE 43 16 620 and DE 43 16 621 that squalene can be extracted from olive oil residues and that extracts from the fruits of the saw-tooth palm can be preserved using carbon dioxide and possibly also with carbon dioxide/propane mixtures. In the two processes a liquid/liquid extraction is carried out in a column with a temperature gradient such that the extraction gas is present near the bottom in a subcritical state and in a supercritical state at the top of the column in order to achieve an effective separation. The necessary process pressure is between 40 and 350 bar and is preferably between 90 and 160 bar. Preferred process temperatures are stated as being 20.degree. to 80.degree. C. It is quite apparent that in this process the proximity to the critical state of the extraction gas is a prerequisite for an efficient separation. Thus the process pressures are, however, always considerably higher than with an extraction with subcritical propane which can usually be carried out with pressures of &lt;40 bar.
The object of the invention is therefore to provide a process for fractionating or/and refining natural lipid substances by liquid/liquid extraction with compressed gases which does not have the said disadvantages of the state of the art but instead enables an effective fractionation and refining of natural materials under relatively low pressures and under mild process temperatures so that the cost-effectiveness of the process conditions is also considerably improved.